The control signal may be produced by applying a control voltage to the control input of the appliance. In this context, the control input circuit associated with the control input has the task of recording the control current excited by the control voltage and, if a control signal is present, triggering an appropriate reaction to the control signal in the appliance.
Various control voltages are usual which differ in terms of voltage level and voltage form, particularly DC or AC voltage. In particular, the use of 24 V DC, 48 V DC, 110 V AC, 230 V AC etc. as control voltage is customary. In line with conventional nomenclature, DC stands for DC voltage and AC stands for AC voltage in this context.
To increase the compatibility of an appliance, and at the same time to have to provide the smallest possible number of different control inputs, it is the usual practice to design a control input for a bandwidth including a plurality of control voltages, e.g. for 24 V DC to 48 V DC or 110 V AC to 230 V AC. In this connection, it would be desirable to have a control input which is equally suitable for all common voltage levels and also for DC and AC voltages.
A conventional control input circuit with a purely nonreactive input characteristic can generally fulfill this not at all or only inadequately, especially since a rising control voltage usually means that it is also necessary to take account of a rising residual current which inevitably flows through the control line connecting the appliance to the control unit even when there is currently no control signal applied. In addition, this residual current is overlaid by interference currents which are transferred to the control line, particularly through capacitive interference injection. Such interference injection may arise, by way of example, when the control line is placed close to a network cable with only inadequate shielding. A control input circuit designed for wideband control voltages must imperatively have a comparatively low trigger threshold in order to trigger with certainty when a low control voltage is used.
On the other hand, however, a control input circuit with a nonreactive characteristic has an increased risk of the trigger threshold being exceeded. This is merely on account of the residual current and/or the interference current when a high control voltage is used.
To increase the wide bandwidth of a control input, from time to time a control input circuit is also used which contains a constant current sink. The term constant current sink is understood to refer to, for example, a circuit module whose drawn current is largely independent of the applied voltage within a prescribed voltage range. In the case of a control input circuit of the latter type, however, it is usually possible to set only a comparatively small drawn current in order to limit the maximum power loss arising in the control input circuit and the associated heating of the appliance to a tenable degree. The maximum drawn current which can be set without the need to provide complex overheating protection is in this case often too small to reach the minimum turned-on current demanded in accordance with the relevant standards, e.g. EN 61131-2: 1994.